US10044164B2 - Laser repetition rate multiplier and flat-top beam profile generators using mirrors and/or prisms - Google Patents
Laser repetition rate multiplier and flat-top beam profile generators using mirrors and/or prisms Download PDFInfo
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10038—Amplitude control
- H01S3/10046—Pulse repetition rate control
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- G—PHYSICS
- G03—PHOTOGRAPHY; CINEMATOGRAPHY; ANALOGOUS TECHNIQUES USING WAVES OTHER THAN OPTICAL WAVES; ELECTROGRAPHY; HOLOGRAPHY
- G03F—PHOTOMECHANICAL PRODUCTION OF TEXTURED OR PATTERNED SURFACES, e.g. FOR PRINTING, FOR PROCESSING OF SEMICONDUCTOR DEVICES; MATERIALS THEREFOR; ORIGINALS THEREFOR; APPARATUS SPECIALLY ADAPTED THEREFOR
- G03F7/00—Photomechanical, e.g. photolithographic, production of textured or patterned surfaces, e.g. printing surfaces; Materials therefor, e.g. comprising photoresists; Apparatus specially adapted therefor
- G03F7/70—Microphotolithographic exposure; Apparatus therefor
- G03F7/70008—Production of exposure light, i.e. light sources
- G03F7/70025—Production of exposure light, i.e. light sources by lasers
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
- H01S3/0057—Temporal shaping, e.g. pulse compression, frequency chirping
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08004—Construction or shape of optical resonators or components thereof incorporating a dispersive element, e.g. a prism for wavelength selection
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08054—Passive cavity elements acting on the polarization, e.g. a polarizer for branching or walk-off compensation
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/08059—Constructional details of the reflector, e.g. shape
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/05—Construction or shape of optical resonators; Accommodation of active medium therein; Shape of active medium
- H01S3/08—Construction or shape of optical resonators or components thereof
- H01S3/081—Construction or shape of optical resonators or components thereof comprising three or more reflectors
- H01S3/083—Ring lasers
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
- H01S3/10061—Polarization control
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S2301/00—Functional characteristics
- H01S2301/20—Lasers with a special output beam profile or cross-section, e.g. non-Gaussian
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- H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
- H01S2301/00—Functional characteristics
- H01S2301/20—Lasers with a special output beam profile or cross-section, e.g. non-Gaussian
- H01S2301/203—Lasers with a special output beam profile or cross-section, e.g. non-Gaussian with at least one hole in the intensity distribution, e.g. annular or doughnut mode
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- H01S2301/00—Functional characteristics
- H01S2301/20—Lasers with a special output beam profile or cross-section, e.g. non-Gaussian
- H01S2301/206—Top hat profile
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/005—Optical devices external to the laser cavity, specially adapted for lasers, e.g. for homogenisation of the beam or for manipulating laser pulses, e.g. pulse shaping
- H01S3/0071—Beam steering, e.g. whereby a mirror outside the cavity is present to change the beam direction
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- H01S3/00—Lasers, i.e. devices using stimulated emission of electromagnetic radiation in the infrared, visible or ultraviolet wave range
- H01S3/10—Controlling the intensity, frequency, phase, polarisation or direction of the emitted radiation, e.g. switching, gating, modulating or demodulating
Abstract
A repetition rate (pulse) multiplier includes one or more beam splitters and prisms forming one or more ring cavities with different optical path lengths that delay parts of the energy of each pulse. A series of input laser pulses circulate in the ring cavities and part of the energy of each pulse leaves the system after traversing the shorter cavity path, while another part of the energy leaves the system after traversing the longer cavity path, and/or a combination of both cavity paths. By proper choice of the ring cavity optical path length, the repetition rate of an output series of laser pulses can be made to be a multiple of the input repetition rate. The relative energies of the output pulses can be controlled by choosing the transmission and reflection coefficients of the beam splitters. Some embodiments generate a time-averaged output beam profile that is substantially flat in one dimension.
Description
The present application is a divisional of U.S. patent application Ser. No. 14/596,738, entitled “Laser Repetition Rate Multiplier And Flat-Top Beam Profile Generators Using Mirrors And/Or Prisms” which claims priority to U.S. Provisional Patent Application Ser. No. 62/015,016, entitled “Laser Pulse Multiplication Using Prisms”, filed on Jun. 20, 2014, and incorporated by reference herein, also claims priority to U.S. Provisional Patent Application Ser. No. 62/038,471 entitled “Laser Repetition Rate Multiplier and Flat-Top Beam Profile Generators”, filed on Aug. 18, 2014, and incorporated by reference herein.
This application is related to U.S. patent application Ser. No. 13/487,075 entitled “Semiconductor Inspection And Metrology System Using Laser Pulse Multiplier” and filed on Jun. 1, 2012 by Chuang et al., to U.S. Pat. No. 9,151,940 entitled “Semiconductor Inspection and Metrology System Using Laser Pulse Multiplier” and issued on Oct. 6, 2015 by Chuang et al., to U.S. patent application Ser. No. 14/455,161, entitled “Split Gaussian Beams and Multi-Spot Flat-Top Illumination for Surface Scanning Systems” and filed on Aug. 8, 2014 by Chuang et al. All of these applications are incorporated herein by reference.
Field of the Invention
The present invention relates to reducing the optical peak power of laser pulses in the temporal domain and, optionally, to homogenizing the beam power distribution in a spatial domain. This peak power reduction and homogenization system may use curved mirrors, beam splitters, wave plates, and prisms to generate an optimized pulse repetition-rate multiplier with a flat-top spatial power distribution profile. The present invention is particularly useful in semiconductor inspection and metrology systems.
Related Art
The illumination needs for inspection and metrology are generally best met by continuous wave (CW) light sources. A CW light source has a constant power level, which allows for images or data to be acquired continuously. However, at many wavelengths of interest, particularly ultraviolet (UV) wavelengths, CW light sources of sufficient radiance (power per unit area per unit solid angle) are not available, are expensive or are unreliable. If a pulse laser is the only available, or cost-effective, light source with sufficient time-averaged radiance at the wavelength of interest, then using a laser with a high repetition rate and wide pulse width is best. The higher the pulse repetition rate, the lower the instantaneous peak power per pulse for the same time-averaged power level. The lower peak power of the laser pulses results in less damage to the optics and to the sample or wafer being measured, as most damage mechanisms are non-linear and depend more strongly on peak power rather than on average power.
In inspection and metrology applications, an additional advantage of an increased repetition rate is that more pulses are collected per data acquisition or per pixel leading to better averaging of the pulse-to-pulse variations and improved signal-to-noise ratios. Furthermore, for a rapidly moving sample, a higher pulse rate may lead to a better sampling of the sample position as a function of time, as the distance moved between each pulse is smaller.
The repetition rate of a laser subsystem can be increased by improving the laser medium, the pump system, and/or its driving electronics. Unfortunately, modifying a UV laser that is already operating at a predetermined repetition rate can require a significant investment of time and money to improve one or more of its constituent elements, and may only improve the repetition rate by a small increment. Furthermore increasing the repetition rate of the fundamental laser in a UV laser reduces the peak power of the fundamental. This reduces the efficiency of the frequency conversion (which is necessarily a non-linear process) and so makes it harder to generate high average UV power levels.
In many inspection applications, a flat or uniform, rather than Gaussian, illumination profile is desired. Spatially uniform illumination on the sample results in a more uniform signal-to-noise ratio across the illuminated area and a higher dynamic range compared with non-uniform illumination. Although incoherent light sources may be able to more readily generate uniform illumination than the Gaussian profile of a laser, such light sources have much broader bandwidth (complicating the optical design because of chromatic aberration) and lower power density (reducing signal-to-noise ratios) than a laser can provide. One known way to achieve an approximately flat profile from a Gaussian laser beam is to crop off the Gaussian tails and only use the central region (close to peak) of the beam. This method is simple to apply; however, if a reasonably flat profile is required a large fraction of the laser power is cropped off and wasted. For example, if the maximum intensity variation in the illumination is required to be about 10%, then about 65% of the power is wasted, and a 20% variation requires wasting approximately 50% of the power.
Therefore, a need arises for a practical, inexpensive technique to improve the repetition rate of a UV laser that operates on the output of the laser. Furthermore it would be advantageous if the optical subsystem that increases the repetition rate can be compact so that it can readily be incorporated into a system without taking up a lot of space. Still furthermore there is a need for a repetition rate multiplier that can generate an approximately flat output profile while adding no, or few, additional components to the repetition rate multiplier, thus saving space and minimizing optical power losses.
A system for inspecting or measuring a sample is described. This system includes an illumination source, a device configured to perform light detection, optics configured to direct light from the illumination source to the sample and to direct light outputs, reflections, or transmissions from the sample to a sensor. Notably, the illuminator comprises a pulsed laser emitting an ultra-violet (UV) wavelength (i.e. a wavelength shorter than about 400 nm) and a repetition rate multiplier that multiplies the repetition rate of the pulses from the pulsed laser. The repetition rate multiplier increases the number of laser pulses per unit time and decreases the peak power of each laser pulse. The decreased peak power reduces or eliminates damage to the system optics or the sample being inspected or measured, and allows use of a higher average laser power level for a given damage threshold, thus improving the signal-to-noise ratio and/or the speed of the inspection or measurement. Multiplying the repetition rate after generation of the UV harmonics maintains the efficiency of the UV harmonic conversion because the peak power of the laser pulses is not reduced in the harmonic conversion chain.
Inspection and measurement systems incorporating a repetition rate multiplier as described herein are particularly useful at deep UV (DUV) wavelengths, i.e. wavelengths shorter than about 300 nm, and vacuum UV (VUV) wavelengths, i.e. wavelengths shorter than about 190 nm, as high peak power levels at these wavelengths can quickly damage many different kinds of materials.
The sample may be supported by a stage, which moves the sample relative to the optics during the inspection or measurement.
The exemplary inspection or measurement system may include one or more illumination paths that illuminate the sample from different angles of incidence and/or different azimuth angles and/or with different wavelengths and/or polarization states. The exemplary inspection or measurement system may include one or more collection paths that collect light reflected or scattered by the sample in different directions and/or are sensitive to different wavelengths and/or to different polarization states.
Inspection and measurement systems incorporating a repetition rate multiplier may be further configured to generate a time-averaged spatially uniform beam profile (i.e. a flat-top profile). An inspection or measurement system incorporating a multiplier and flat-top profile generators described herein provide two or more times multiplication of the laser repetition rate and a more uniform time-averaged beam profile using a small number of optical components in a compact space. The inspection and metrology systems described herein are capable of using higher average laser power enabling a higher throughput, better signal quality, and more efficient use of the laser energy.
Method and systems for multiplying the repetition rate of a pulsed laser are described. These methods and systems split an input laser pulse into multiple pulses separated in time so as to multiply the laser repetition rate by an integer such as 2, 3 or 4. An incoming pulse is split into two so that part of the pulse continues on, and part of the pulse enters a ring cavity. After traversing at least a section of the ring cavity, the pulse is split again and part of the pulse leaves the ring cavity and part continues on in the ring cavity. The repetition rate multiplier may be further configured to generate a time-averaged output profile that is approximately flat in one dimension and substantially Gaussian in the perpendicular dimension. The repetition rate multiplier may comprise flat mirrors, curved mirrors, polarized beam splitters, wave-plates, beam compensators and/or lenses.
In one exemplary embodiment, an input laser pulse is split into two by a wave plate and a polarized beam splitter. One part of the input laser pulse is directed around a short ring cavity loop and the other part is directed around a long ring cavity loop. On their way back to the input/output coupler (which may comprise a polarized beam splitter), the pulses encounter another wave plate which determines the fraction of the pulse energy that leaves the cavities. The remaining fraction of the pulse energy traverses again the cavities.
In one exemplary embodiment the short and long cavity loop lengths are respectively set to be ⅓ and ⅔ of the input laser pulse-to-pulse spatial separation so that the output pulses will be delayed in time by ⅓, ⅔ or an integer multiple of ⅓ of the pulse-to-pulse period. These delayed pluses form a pulse train with a repetition rate that is three times that of the original input laser pulses. The orientation and retardance of the two wave plates may be chosen such that the output pulses have substantially equal pulse-to-pulse energy.
In another exemplary embodiment, which can also triple the repetition rate, two mirrors form a ring cavity and two beam splitters are placed in between. Whenever a pulse goes through a beam splitter, it splits into two pulses; one of the pulses goes straight through while the other is deflected. With these two beam splitters, some pulses traverse a longer cavity loop while others traverse a shorter one. In one exemplary embodiment, the shorter loop has a path length approximately equal to about ⅓ of the original input pulse-to-pulse separation, and the longer loop path length is approximately ⅔ of the pulse-to-pulse separation. In this embodiment, the output pulses form a pulse train with a repetition rate that is three times that of the original input pulses. With an appropriate choice of mirror curvature, mirror separation, and beam-splitter reflectivities, the output pulses can have substantially equal pulse-to-pulse energy.
In one embodiment, two beam compensators comprising flat plates are place in the cavity to substantially compensate for the shifts in the beam path caused by the beam splitters so that the beams reflect from the mirrors in a pattern that is substantially symmetric about the cavity axis. In another embodiment, one (or both) beam compensators are replaced by a prism (or prisms) that substantially compensates for the shift in beam path of one (or both) beam splitters. In yet another embodiment, no prism or beam compensator is used and the beam splitters are positioned in such a way that each compensates for the beam shift caused by the other.
In yet another exemplary embodiment, flat mirrors and prisms are inserted into the light path within the cavity to form a secondary cavity loop between the same pair of curved mirrors with a loop length about half that of the primary cavity loop. If the primary cavity loop length is set to about half of the original input pulse-to-pulse separation, the primary cavity loop can double the pulse repetition rate. Pulses leaving the primary cavity loop enter the secondary cavity loop which has a length that is about half that of the primary cavity loop, thus doubling the pulse repetition rate again, resulting in an output pulse repetition rate that is four times that of the input laser.
Some embodiments use a prism such as an isosceles triangle prism or a Dove prism to double the number of round trips that the beam makes within the cavity. The two cavity routes generate two parallel output beams. The deviation between these two beams can be chosen such that they overlap to form a time-averaged spatially approximately flat-top beam profile.
In one embodiment, a 2× pulse multiplier scheme is used as a base for flat-top profile generation. In another embodiment, a 3× pulse multiplier is used as a base for flat-top profile generation. In yet another embodiment, the abovementioned 4× pulse multiplier scheme is used as a base for flat-top profile generation. This embodiment can generate four parallel beams with a predetermined power ratio between them. By selecting the separations between the beams and the power ratios between them, a wider flatter time-averaged beam profile can be achieved. Any of these repetition-rate multipliers that generate an approximately flat-top profile may comprise beam compensators and/or prisms.
In one embodiment, the ring cavity comprises right angle mirror pairs. In another embodiment, the ring cavity comprises prisms that utilize total internal reflection to achieve high reflectivity. With an appropriate prism design, a ring cavity using prisms can achieve low losses without using high reflectivity coatings. High reflectivity coatings can easily be damaged by high intensity laser pulses, particularly at short wavelengths, so many of the methods and systems described herein can have a longer operating life and/or lower maintenance costs compared with other ring cavities especially when used for multiplying the repetition rate of DUV and VUV lasers.
In one embodiment one or more prisms in the ring cavity are designed so that the angle of incidence of the laser beam entering and exiting the prism is close to Brewster's angle and the laser beam is substantially P-polarized relative to the prism entrance and exit surfaces, so that the losses due to reflection are kept small without using any anti-reflection (AR) coating. AR coatings can easily be damaged by high intensity laser pulses, particularly at short wavelengths, so this embodiment can have a longer operating life and/or lower maintenance costs compared with ring cavities using AR coatings especially when used for multiplying the repetition rate of DUV and VUV lasers.
In one embodiment, the Brewster cut of the prism(s) is oriented for the beam polarization lies in the same ring-cavity plane, while in another embodiment the Brewster cut of the prism(s) is oriented for the beam polarization that is perpendicular to the ring-cavity plane.
In one embodiment, the beam is totally internally reflected twice in a single prism of appropriate design. Such a prism can replace two folding mirrors in a ring cavity, and so reduce the total number of components and simplify the process of aligning the ring cavity.
In one embodiment a right-angled prism is used in the ring cavity. It reflects the beam twice which sends the beam back in the opposite direction while also displacing it in space. This unique feature of the right-angled prism results in the flexibility to multiply the effective cavity length by simply rotating the right-angle prism to a specific angle. For example, two ring cavities may be constructed with similar physical lengths, but with one having an optical path length that is an integer multiple of (such as twice) the optical path length of the other so that the two ring cavities may be cascaded in order to multiply the pulse repetition rate by a larger factor then can be conveniently achieved in a single ring cavity.
In a preferred embodiment, the fraction of the energy of each input laser pulse that is directed into the ring cavity is controlled by selecting the angle of incidence and polarization relative to a surface to achieve the desired reflection and transmission coefficients. This has the advantage of avoiding the need for any coating on the surface and so avoids the possibility of coating damage caused by the peak power density of the laser pulses, which can be a problem particularly when the laser repetition rate multiplier is used with a deep UV or vacuum UV laser with an average power of hundreds of mW or greater. Such lasers are increasingly needed in semiconductor inspection and metrology systems in order to achieve the desired sensitivity and signal-to-noise ratio when inspecting or measuring features with dimensions of about 100 nm or smaller.
In a preferred embodiment, one or more lenses are used within the ring cavity to re-image each laser pulse such that it retains approximately the same shape and size each time it traverses the cavity. One embodiment uses Brewster's angle lenses without coatings to refocus each laser pulse, thus avoiding the risk of coating damage.
In a preferred embodiment, two or more of the above described features are combined in one laser repetition rate multiplier. For example, in one preferred embodiment a laser repetition rate multiplier comprises a ring cavity comprising three uncoated prisms wherein the laser beam inside the cavity is substantially p polarized relative to the surfaces of those prisms. Two of the prisms use total internal reflection to circulate the laser beam efficiently within the ring cavity. A third prism has a surface that is oriented so that the laser beam is approximately s polarized relative to that surface and the input pulses are incident at an angle chosen so that desired fraction each input pulse is directed into the ring cavity.
Wafer inspection systems, patterned wafer inspection systems, photomask inspection systems, and metrology systems incorporating a laser pulse multiplier are described. The compact size of the laser pulse multipliers described herein makes them relatively easy to incorporate into inspection and metrology systems. The use of uncoated optics in the laser pulse multiplier allows those inspection and metrology systems to operate with high powered deep UV lasers without performance degradation or maintenance issues due to coating damage.
Improved illumination systems for semiconductor inspection and measurement systems are described herein. The following description is presented to enable one of ordinary skill in the art to make and use the invention as provided in the context of a particular application and its requirements. Various modifications to the described embodiments will be apparent to those with skill in the art, and the general principles defined herein may be applied to other embodiments. Therefore, the present invention is not intended to be limited to the particular embodiments shown and described, but is to be accorded the widest scope consistent with the principles and novel features herein disclosed.
An illumination source 102 comprises one or more pulsed lasers and a repetition rate multiplier as described herein. Illumination source 102 may emit DUV and/or VUV radiation. Optics 103, including an objective lens 105, directs that radiation towards and focuses it on sample 108. Optics 103 may also comprise mirrors, lenses, and/or beam splitters (not shown in detail for simplicity). Light reflected or scattered from sample 108 is collected, directed, and focused by optics 103 onto a detector 106, which is within a detector assembly 104.
One embodiment of inspection system 100 illuminates a line on sample 108, and collects scattered and/or reflected light in one or more dark-field and/or bright-field collection channels. In this embodiment, detector 106 may include a line sensor or an electron-bombarded line sensor. In this embodiment repetition rate multiplier 120 within illumination source 102 may be configured to generate a flat-top profile so as to efficiently generate a substantially uniform line illumination.
Another embodiment of inspection system 100 illuminates multiple spots on sample 108, and collects scattered and/or reflected light in one or more dark-field and/or bright-field collection channels. In this embodiment, detector 106 may include a two-dimensional array sensor or an electron-bombarded two-dimensional array sensor.
Additional details of various embodiments of inspection system 100 are described in U.S. Published Application 2013/0016346 entitled “Wafer inspection system”, published on Jan. 17, 2013, U.S. Pat. No. 7,957,066 entitled “Split Field Inspection System Using Small Catadioptric Objectives”, issued on Jun. 7, 2011, U.S. Pat. No. 7,345,825 entitled “Beam Delivery System For Laser Dark-field Illumination in a Catadioptric Optical System”, issued on Mar. 18, 2008, U.S. Pat. No. 5,999,31, entitled “Ultra-Broadband UV Microscope Imaging System With Wide Range Zoom Capability”, issued on Dec. 7, 1999, and U.S. Pat. No. 7,525,649 entitled “Surface Inspection System Using Laser Line Illumination With Two Dimensional Imaging”, issued on Apr. 28, 2009. All of these patents are incorporated by reference herein.
More details of inspection systems in accordance with the embodiments illustrated in FIGS. 2A and 2B are described in U.S. Pat. No. 7,525,649, entitled “Surface inspection system using laser line illumination with two dimensional imaging”, issued on Apr. 28, 2009, and U.S. Pat. No. 6,608,676, entitled “System for detecting anomalies and/or features of a surface”, issued on Aug. 19, 2003. Both of these patents are incorporated by reference herein.
In an oblique illumination channel 312, the second polarized component is reflected by a beam splitter 305 to a mirror 313 which reflects such beam through a half-wave plate 314 and focused by optics 315 to sample 309. Radiation originating from the oblique illumination beam in oblique channel 312 and scattered by sample 309 is collected by paraboloidal mirror 310 and focused to sensor 311. Sensor 311 and the illuminated area (from the normal and oblique illumination channels on sample 309) are preferably at the foci of paraboloidal mirror 310.
U.S. Pat. No. 6,201,601, entitled “Sample Inspection System”, issued on Mar. 13, 2001, and U.S. Published Application 2013/0016346 entitled “Wafer Inspection System”, published on Jan. 17, 2013 by Romanovsky et al., both of which are incorporated by reference herein, describe inspection system 300 in further detail.
In a dark-field mode, adaptation optics 402 control the laser illumination beam size and profile on the surface being inspected. A mechanical housing 404 includes an aperture and window 403, and a prism 405 to redirect the laser along the optical axis at normal incidence to the surface of a sample 408. A prism 405 also directs the specular reflection from surface features of sample 408 out of an objective 406. Objective 406 collects light scattered by sample 408 and focuses it on a sensor 409. Lenses for objective 406 can be provided in the general form of a catadioptric objective 412, a focusing lens group 413, and a tube lens section 414, which may, optionally, include a zoom capability.
In a bright-field mode, a broad-band illumination module 420 directs broad-band light to a beam splitter 410, which reflects that light towards focusing lens group 413 and catadioptric objective 412. Catadioptric objective 412 illuminates sample 408 with the broadband light. Light that is reflected or scattered from the sample is collected by objective 406 and focused on sensor 409. Broad-band illumination module 420 comprises, for example, a laser-pumped plasma light source or an arc lamp. Broad-band illumination module 420 may also include an auto-focus system to provide a signal to control the height of sample 408 relative to catadioptric objective 412.
U.S. Pat. No. 7,245,825 entitled “Beam Delivery System For Laser Dark-Field Illumination in a Catadioptric optical system”, issued on Mar. 18, 2008 and incorporated by reference herein, describes system 400 in further detail.
In one preferred embodiment, the optical path length of loop A is set to approximately one-third of the pulse-to-pulse distance of input laser pulses, and path length of loop B is set to approximately twice that of loop A. This results in output pulses at approximately one third and two thirds of the time interval between input laser pulses and approximately coincident with the input pulses, thus tripling the repetition rate of the laser. In this embodiment, preferably the angles α and β of the principal axes of wave plates A07 and A08 respectively are set to be either approximately α=29° and β=16°, respectively, or approximately α=16° and β=29°, respectively, so as to produce approximately equal energy in each output pulse. If small differences (such as a few percent) in the energy of each pulse are acceptable in a specific application, then angles that differ from these values by one or two degrees may be acceptable. Lenses (not shown) may be incorporated into the dual cavity and/or one of more of mirrors A04, A05 and A06 may be curved such that the Gaussian beam waist and size of each pulse is re-imaged to the same condition when it returns to the same position.
Repetition rates other than three are possible with this dual cavity. For example Loop A may be set to have an optical path length equal to approximately one quarter of the separation of input pulses, and Loop B may be set to be approximately twice the length of Loop A. This would result in quadrupling the repetition rate of the input laser pulses. However such a scheme cannot generate equal output pulse energies and so would only be useful when equal output pulse energies are not required.
Laser input pulses (input) arrive at beam splitter B03. Part of the energy of each pulse is reflected from beam splitter B03 to point B07 on curved mirror B01, then to point B08 on curved mirror B02, through beam splitter B04 to point B09 on curved mirror B01, then to point B10 on curved mirror B02, and back to beam splitter B03. The other part of the energy of each pulse is transmitted through beam splitter B03, to beam splitter B04 where it is reflected to point B09 on curved mirror B01, then to point B10 on curved mirror B02, and back to beam splitter B03. In preferred embodiments, the optical path length of the shorter loop (B03-B04-B09-B10-B03) is approximately half of that of the longer one (B03-B07-B08-B04-B09-B10-B03). When the distance between the two curved mirrors B01 and B02 is approximately one-sixth of the original pulse-to-pulse spatial separation of input laser beam, the output pulse train will have triple the repetition rate of the input pulses. Beam compensators B05 and B06 have optical thicknesses and orientations chosen so as to substantially compensate for the displacement of the laser beam within the cavity caused by beam splitters B03 and B04 respectively.
Similar to the output of the 2× repetition rate multiplier described in the above-cited '940 patent and illustrated in FIGS. 2A and 2B of that application, the output of pulse repetition rate tripler 120B consists of a series of pulse trains, each pulse train comprising a series of pulses that have traversed one or both of the cavities one or more time. Pulse repetition rate tripler 120B has three output pulse trains per input pulse compared with two output pulses per input pulse for the 2× repetition rate multiplier. In a preferred embodiment of the pulse repetition rate tripler, the total energies in each output pulse train are made approximately equal to one another by setting the reflectivities of beam splitters B03 and B04 to be approximately equal to
i.e. approximately 0.28 and approximately 0.72. Note that either B03 can have a reflectivity of approximately 0.28 and B04 can have a reflectivity of approximately 0.72, or B04 can have a reflectivity of approximately 0.28 and B03 can have a reflectivity of approximately 0.72. Both configurations produce substantially equal output pulse energies. Since pulse-to-pulse energy variations of a few percent may be acceptable in many inspection applications, the beam splitter reflectivities may be chosen to have values that differ by a few percent from 0.28 and 0.72. As one skilled in the relevant arts understands, the reflectivity of a beam splitter can be controlled by the selection of the beam splitter material, the thickness(es) and material(s) of any layer or layers coated on the surface, and the angle of incidence on the beam splitter.
A special feature of this scheme is that this secondary cavity loop, which further multiplies the repetition rate a second time, utilizes same set of curved mirrors (B01 and B02) as the first cavity loop. In addition, the flat mirrors D05 and D07 can be combined into one optical element with high reflectivity (HR) coatings on both sides. These features give rise to a more compact footprint as compared with a setup comprising two individual 2× pulse multipliers cascaded together. Note that, though convenient, it is not required to combine mirrors D07 and D05, and the beam D03 may be directed along a different path from that shown to arrive at beam splitter D06. Alternative layouts are possible and are within the scope of this embodiment.
Each time a laser pulse encounters beam splitter B03, part of the energy of the pulse gets reflected and exits the system. A pulse traveling on route E02 generates a pulse on exit route E04, and a pulse on route E03 generates a pulse on exit route E05. With this setup, one Gaussian beam splits into two spatially. By controlling the separation between E04 and E05, the degree of overlap between these two laser beams can be controlled. In a preferred embodiment the output beam profile possess an approximately flat-top time-averaged intensity, as illustrated in FIG. 12 . An approximately flat top output beam profile can be created by displacing one Gaussian with respect to the other by approximately 0.5 times the beam waist radius (i.e. the radius at which the beam power density is 1/e2 of its peak value, or equivalently the radius at which beam amplitude is 1/e of its peak value). This flat-top profile is very desirable in many applications which a homogenized spatial power distribution is required. Note that, because laser pulses on paths E04 and E05 leave the cavity at times separated by much longer than a duration of an individual pulse (such as separated in time by approximately half the time interval between input laser pulses), there is no interference of one pulse with another resulting in the desired relatively flat-top profile. Interference between the two displaced Gaussians, which could occur without a long enough time delay between the pulses, could cause a non-flat top of the profile.
The above-mentioned repetition rate multipliers 120E-1 and 120E-2, which facilitate the flat-top schemes shown in FIG. 11A and FIG. 11B , are based on a scheme that doubles the repetition rate (e.g., the arrangement described above with reference to FIG. 9 ). Therefore, it has the advantage that one optical cavity not only spreads out the laser pulse energy distribution in the time domain but also homogenizes the energy distribution in the spatial domain.
where RB03 and RB04 are the reflectivities of beam splitter B03 and B04 respectively. Preferably the thicknesses of both beam splitters and the beam compensator are all equal so that it is straightforward to align the optics to achieve two closed loops within the optical cavity.
Alternatively, FIGS. 14A and 14B illustrate exemplary flat-top beam generators 102G-1 and 102G-2 according other embodiments that utilize repetition rate multipliers 120G-1 and 120G-2, respectively, having configurations similar to FIG. 13 but without using any beam compensator or prism, which receive input laser pulses generated by lasers 119G-1 and 119G-2, respectively, and to generate output laser pulses (output) having flat-top beam profiles. With proper arrangement of the beam splitter positions (e.g., by moving beam splitter B04-1 to the right as indicated in FIG. 14A , or by moving beam splitter B04-2 to the left as indicated in FIG. 14B ), the generation of flat-top beam profiles without using any compensator or prism is possible in 3× multiplier-based scheme. In addition, the coatings of beam splitters can be either facing to different directions (FIG. 14A ) or facing toward the same direction (FIG. 14B ). You can also view the embodiments in FIGS. 14A and 14B as derivations of the arrangements shown in FIGS. 7A and 7B with one of the beam splitters offset in location, which causes the beam to be split into two and, hence with the appropriate offset, generate a time-averaged flat-top output profile.
In order to generate a flat-top beam ensemble, three parameters need to be arranged in an appropriate relation. By proper adjusting the shifting distance of prism D02, one can tune the space (a) between D03 and H01, and hence the space between H02 and H03, and the space between H04 and H05 as well. By adjusting the displacement of prism D04, one can tune the space between H03 and H04 (b). Lastly, one can choose the beam splitter D01 reflectivity such that beams D03 and H01 possess different powers with a desired ratio.
In the above cited Ser. No. 13/487,075 application and '940 patent which are incorporated by reference herein, alternative embodiments of laser pulse repetition rate multipliers are described. These applications explain how the repetition rate of a pulsed laser may be doubled using a ring cavity of appropriate length by using a beam splitter to direct approximately ⅔ of the energy of each input pulse into the cavity while directing approximately ⅓ of the energy of each pulse to the output. With a cavity optical path length corresponding to approximately half the time interval between the input laser pulses, the output pulse trains form envelopes of substantially similar energy which repeat at a repetition rate that is twice that of the original laser pulses. The '940 patent also describes how to adjust the transmission and reflectivity of the beam splitter so as to maintain substantially equal output pulse energies to compensate for losses in the beam splitter and ring cavity. Any of the principles described in the Ser. No. 13/487,075 application and '940 patent may be applied as appropriate to the various embodiments of pulse repetition rate multipliers described herein.
A detailed description of one or more embodiments of the invention is provided above along with accompanying figures that illustrate the principles of the invention. The invention is described in connection with such embodiments, but the invention is not limited to any embodiment.
For example, in one embodiment, the optical components can be coated with appropriate coatings for the laser wavelength. Each surface of any transmissive elements, such as wave-plates, can also have an anti-reflection coating that minimizes the amount of laser energy reflected at each surface. Mirrors can be coated with coatings designed to maximize the reflection and minimize scattering at the laser wavelength.
In another example, in one embodiment, the Herriott-cell-like cavity may have a different shape or a different number of mirrors from the examples given above.
Although the above illustrated embodiments are drawn in one plane, alternative embodiments can place one of the cavity loops, such as the secondary cavity loop in FIG. 8 or FIG. 15A , in a plane approximately perpendicular, or rotated relative, to the plane of another cavity loop, such as the primary cavity loop, while still using the same set of mirrors. For example, FIGS. 16A and 16B are front and side views showing a 2× pulse repetition rate multiplier arrangement in which input pulses and output pulses are directed in a plane perpendicular to the plane of the cavity loop formed by beam splitter D01, beam compensator or prism D02, and curved mirrors B01 and B02. FIGS. 16C and 16D are front and top views showing a 4× arrangement in which one or more mirrors or prisms can be used outside of the cavity to direct or deflect the light from prism D04 to mirror D05 lying in the plane of the secondary loop. Note in FIG. 16D , the input and output beams, the plane of the 2× cavity loop, beam splitter D01 and beam compensator or prism D02 are shown as seen from above by dashed lines for reference. The optical components of one cavity loop are positioned so that they do not intercept the other cavity loop. An advantage of placing different cavity loops in different planes is that it is possible for each of the cavity loops to be reflected from the curved mirrors, such as mirrors B01 and B02, at substantially similar distances from the centers of those mirrors (as shown in FIG. 16C , where the dashed lines show the input and output light paths and the plane of the 2× loop as viewed from the front), allowing the cavity loops to be in focus at the same time and so minimize aberrations of the laser beam as it traverses the cavity loops multiple times. By orienting and displacing prisms D02 and D04 in appropriate directions in a manner similar to that illustrated in FIG. 15A , the 4× laser pulse repetition rate multiplier shown in FIGS. 16C and 16D can generate an output with a time-averaged substantially flat-top profile similar to that shown in FIG. 15A .
As explained in the '940 patent, in a laser pulse repetition rate doubler, if it is desired that the each output pulse be of substantially equivalent total energy, then the beam splitter I01 should be designed to reflect a substantially ⅔ (second) fraction of the energy of each input laser pulse into the ring cavity, and to transmit a substantially ⅓ (first) fraction of each input laser pulse such that the ⅓ fraction exits repetition rate multiplier 120I at a first time, and such that the ⅔ fraction exits repetition rate multiplier 120I at a second time after being reflected between reflective elements I02 and I03. This can be achieved, for example, by the use of an appropriate coating on beam splitter I01. Note that if substantially equal output pulse energies are not required, then beam splitter I01 may be designed to reflect some fraction of each laser pulse other than ⅔. For example, when used in an inspection system, it may be desirable that each output pulse have substantially equal peak power to allow operation close to the damage threshold of the article being inspected. Substantially equal peak powers of the output pulses can be achieved using a beam splitter than reflects about 62% of each laser pulse into the ring cavity.
As explained in the '940 patent, the optical path length of the ring cavity may be set to be slightly longer than, or slightly shorter than, the distance equivalent to half the time interval between input laser pulses so as to broaden the output pulses and reduce the peak power of each output pulse and so reduce the possibility of damage to down-stream optics or to an article being inspected or measured by a system incorporating a laser repetition rate multiplier.
For example if the input laser pulses have a repetition rate of 120 MHz, then an ring cavity optical path length of about 1.249 m would result in the repetition rate being doubled with the output pulses being approximately equally spaced in time. To achieve this ring cavity optical path length, the physical distance between the prisms would need to be about 0.625 m. As understood by one skilled in the appropriate arts, since the laser pulses travel a short distance inside each prism I01 and I02 and the refractive index of the prism material is greater than 1, the optical path length within the prism is a little longer than the physical distance traveled by the laser pulses inside the prism. An appropriate small adjustment can be made to the physical distance between the prisms in order to compensate for this in order to achieve the desired ring cavity optical path length. If it is desired that the output pulses be broader than the input pulses in order to reduce the peak power of each pulse, then the ring cavity optical path length can be set to be a little longer or a little shorter than 1.249 m, such as a ring cavity optical path length of 1.25 m so that a pulse that has traveled twice around the ring cavity arrives about 6 ps (picoseconds) later than the next input pulse.
In a preferred embodiment, the ring cavity of repetition rate multiplier 120I preferably includes an optical plate I04 to substantially compensate for the offset in the laser beam position caused by the laser pulses passing through beam splitter I01. Optical plate I04 should preferably have an optical thickness substantially equal to the optical thickness of beam splitter I01. Optical plate I04 should preferably be coated with an anti-reflection coating so as to minimize the reflection of laser light from its surfaces. If optical plate I04 is placed in the same arm of the ring cavity (as shown), then preferably optical plate I04 should be oriented at angle to the input laser beam (pulses) that is substantially a mirror image of the angle to laser beam of the beam splitter I01 so as to substantially compensate for the beam displacement caused by the beam splitter I01. If the optical plate I04 is placed in the other arm of the ring cavity (not shown), then it should preferably be oriented substantially parallel to the beam splitter I01.
A key benefit of this design is maintaining substantially the same cavity footprint while changing the ring cavity optical path length by simply rotating one of the prisms.
The two ring cavities can be fabricated from the same optical components. Since the two cavities can have substantially similar external dimensions, much of the mounting hardware and mechanical design can be the same. Space can be used more efficiently than in a design where one ring cavity has about twice the physical length of the other ring cavity.
The geometry of Prism S01 is shown in FIGS. 28, 28A, 28B and 28C . It utilizes Fresnel reflectivity properties as shown in FIGS. 29A and 29B . For S-polarized light hitting a surface of fused silica (or any material with similar refractive index), the reflectivity naturally goes to about 33.3% when angle of incidence is approximately 73°. The input laser beam b1 and the output laser beam b2 are S-polarized with respect to the surface S1. However, when the refracted beam passes through surface S2 or S3, it is P-polarized. If the angle of incidence on these surfaces (S2 and S3) is close to Brewster's angle, the laser beam can pass with minimal loss of power without using any coating and hence avoiding any possibility of laser damage to the coating.
Prism S02 of repetition rate multiplier 120S (see FIG. 27A ) is illustrated in FIGS. 30, 30A and 30B . This prism utilizes reflection at Brewster's angle at surfaces S4 and S5 while using total internal reflection at surface S6.
Prism S03 of repetition rate multiplier 120S (see FIG. 27A ) is illustrated in FIGS. 31, 31A 31B and 31C. It serves as a right-angle prism with two surfaces cut at Brewster's angle. These Brewster's angle cuts are oriented for beam polarization perpendicular to the ring cavity plane, which is different from the prism design P01 in FIG. 25A , which is for polarization parallel to the ring cavity plane.
In a similar manner to that explained above, right-angle prism T03 can be rotated by 90° about a normal direction to the incident surface, to double the beam path length within the cavity.
In the above cited Ser. No. 13/487,075 application and '940 patent which are incorporated by reference herein, alternative embodiments of laser pulse repetition rate multipliers are described. These applications explain how the repetition rate of a pulsed laser may be doubled using a ring cavity of appropriate length by using a beam splitter to direct approximately ⅔ of the energy of each input pulse into the cavity while directing approximately ⅓ of the energy of each pulse to the output. With a cavity optical path length corresponding to approximately half the time interval between the input laser pulses, the output pulse trains form envelopes of substantially similar energy which repeat at a repetition rate that is twice that of the original laser pulses. The '940 patent also describes how to adjust the transmission and reflectivity of the beam splitter so as to maintain substantially equal output pulse energies to compensate for losses in the beam splitter and ring cavity. Any of the principles described in the Ser. No. 13/487,075 application and '940 patent may be applied as appropriate to the various embodiments of pulse repetition rate multipliers described herein.
To reach an even higher repetition rate, one can cascade multiple units of any of the abovementioned laser pulse repetition rate multipliers with each unit having a different cavity length. The output repetition rate can be made equal to 2×, 4×, . . . or 2n× that of the input repetition rate where n is the number of laser pulse repetition rate multiplier cavities, and the optical path length of each cavity is ½, ¼, . . . ½n of the distance between the original pulses.
The above exemplary embodiments illustrate how optical cavities of different lengths may be formed from various combinations of flat mirrors, curved mirrors, prisms and lenses for the purpose of multiplying the repetition rate of a pulsed laser. A repetition rate multiplier or a repetition rate multiplier that generates an output with a time-averaged substantially flat-top profile may be constructed from other combinations without departing from the scope of this invention. For example, a flat mirror may be replaced by a prism (or, in many cases, vice versa), or a curved mirror by a combination of a flat mirror and one, or more, lenses (or vice versa). The choice of which components to use is dictated by many practical considerations, including the laser wavelength, the laser power density at the location of the optical components, the availability of a suitable optical coating for the component, physical space and weight. As explained above, prisms and components with Brewster angle surfaces are generally preferred where power densities are high enough to potentially damage optical coatings.
The scope of the invention is limited only by the claims and the invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the above description in order to provide a thorough understanding of the invention. These details are provided for the purpose of example and the invention may be practiced according to the claims without some or all of these specific details. For the purpose of clarity, technical material and derivation that are known in the technical fields related to the invention has not been described in detail so that the invention is not unnecessarily obscured.
Claims (10)
1. A laser pulse repetition rate multiplier for receiving input laser pulses and for generating output pulses with an output pulse repetition rate that is more than two times that of the input laser pulses, the laser pulse repetition rate multiplier comprising:
a Herriott cell including first and second curved mirrors, first and second beam splitters and at least two fold mirrors forming an optical cavity, and a right-angled prism positioned outside of the optical cavity, the Herriot cell being configured such that portions of each said input pulse are transmitted along a primary cavity loop inside the optical cavity, then passed to the right-angled prism, then transmitted along a secondary cavity loop inside the optical cavity before exiting the optical cavity as one of said generated output pulses,
wherein the two curved mirrors have a radius of curvature approximately equal to an odd integer multiple of one fourth of a spatial separation between said input laser pulses, and
wherein the two curved mirrors have a common radius of curvature and are separated by a distance substantially equal to the radius of curvature.
2. The laser pulse repetition rate multiplier of claim 1 , wherein the Herriot cell is configured such that the output pulse repetition rate is four times that of the original input pulses.
3. The laser pulse repetition rate multiplier of claim 1 ,
wherein the Herriot cell is configured such that at least a portion of each said input laser pulse is directed by the first beam splitter such that it is reflected by first portions of the first and second curved mirrors while on the primary cavity loop, and is directed by the second beam splitter such that it is reflected by second portions of the first and second curved mirrors while on the secondary cavity loop,
wherein said Herriot cell further comprises a prism disposed in the primary cavity loop, and
wherein one of the prism, the at least two fold mirrors, and the right-angle prism is configured to divert laser pulses coming out from the primary cavity loop into the secondary cavity loop.
4. The laser pulse repetition rate multiplier of claim 3 , wherein the Herriot cell is configured such that the primary cavity loop lies in a different plane than the secondary cavity loop.
5. The laser pulse repetition rate multiplier of claim 4 , wherein the first beam splitter receives said input laser pulses and reflects approximately two-thirds of the energy of each said input laser pulse into the primary cavity loop.
6. The laser pulse repetition rate multiplier of claim 1 wherein second beam splitter has a reflectivity of approximately one-third.
7. The laser pulse repetition rate multiplier of claim 1 , wherein the Herriot cell is configured such that:
each said input laser pulse is directed onto the first beam splitter;
the first beam splitter is configured to direct at least a first portion of said each input laser pulse such that the first portion is reflected by the first and second curved mirrors in a first plane while on the primary cavity loop, and then directed toward said right-angle prism;
the right-angle prism is configured to redirect the first portion to the second beam splitter; and
the second beam splitter is configured to direct at least a second portion of each said first portion such that the second portion is reflected by the first and second curved mirrors in a second plane while on the secondary cavity loop, the second plane being different from the first plane.
8. The laser pulse repetition rate multiplier of claim 1 ,
wherein the Herriot cell further comprises one of a beam compensator and prism disposed with the first beam splitter in a first plane, and
wherein the second beam splitter and the at least two fold mirrors are disposed in a second plane.
9. A repetition rate multiplier for receiving input laser pulses and for generating output pulses with an output pulse repetition rate that is at least two times that of the input laser pulses, the laser pulse repetition rate multiplier comprising:
at least one beam splitter and two light reflective elements forming a ring cavity,
wherein said at least one beam splitter is configured to direct a first energy fraction of each input laser pulse such that the first fraction exits the repetition rate multiplier at a first time, and configured to direct a second fraction of the energy of the input laser pulse into the ring cavity such that the second fraction is reflected between the two reflective elements and exits the repetition rate multiplier at a second time,
wherein said two light reflective elements comprise a first curved mirror and a second curved mirror,
wherein said at least one beam splitter includes:
a first beam splitter configured to direct said first and second energy fractions into the ring cavity, and
a second beam splitter configured to direct the second energy fraction of each said input laser pulse out of the repetition rate multiplier after the second energy fraction traverses between the first and second reflective elements at least once;
further comprising:
a prism disposed in the ring cavity,
at least two fold mirrors disposed in the ring cavity; and
a right-angled prism positioned outside of the ring cavity,
wherein the first beam splitter and the prism are configured such that first portions of each said input pulse are transmitted along a primary cavity loop inside the ring cavity,
wherein the second beam splitter and the at least two fold mirrors are configured such that second portions of each said input pulse are transmitted along a secondary cavity loop inside the ring cavity, and
wherein the right-angled prism is configured to direct said first portions leaving the primary cavity loop to the secondary cavity loop.
10. The laser pulse repetition rate multiplier of claim 9 ,
wherein the first beam splitter is configured to direct said first portions of said each input laser pulse such that the first portions are reflected by the first and second curved mirrors in a first plane while on the primary cavity loop,
wherein the right-angle prism is configured to direct the first portions leaving the primary cavity loop to the second beam splitter, and
wherein the second beam splitter is configured to direct the second portions such that the second portions are reflected by the first and second curved mirrors in a second plane while on the secondary cavity loop, the second plane being different from the first plane.
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